When a social network is taken down by a distributed denial of service (DDoS) attack, it’s somewhere between “an inconvenience and real trouble” for an enterprise, according a security expert.

But there are other security breaches at sites like Twitter and Facebook that can be much more dangerous, said Vaclav Vincalek, president of Vancouver network support and security firm Pacific Coast Information Services Ltd.

Both Twitter and Facebook were brought under DDoS attacks Thursday morning.

The Twitter micro-blogging and social networking service was hit with a denial of service attack that rendered the site unavailable for users.

Twitter reported the attack in a post on its blog at about 11a.m.

"We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," the company said in a blog posting by Twitter co-founder Biz Stone.

In a status report about an hour following its acknowledgement of the attack, Twitter reported that the site was back up, but users still were having trouble reaching it. The site itself was down for about two hours before it resumed service, although Twitter remained under attack and warned users in another status update that as it recovered, users would experience "some longer load times and slowness," as well as network timeouts.

The attack on Facebook did not appear to be as severe.

Facebook reported on its own Facebook profile Thursday that users may have had trouble accessing the site or its services because of the attack, but that the situation seemed well in hand by late morning in California, where the company is located.

"We have restored full access for most people," the company reported. "We’ll keep monitoring the situation to make sure you have the reliable experience you expect from us."

Users reported that Facebook was not loading properly or could not be accessed early Thursday, but the site seemed to be working properly by late morning.